The invention relates to protection of differential input terminals of amplifiers and comparators, and in particular, to an amplifier of differential type having protected input terminals.
In many practical circuits comprising an operational amplifier, the voltage between the two input terminals of such an amplifier is very close to zero due to the fact that feedback is used and that the gain of an operational amplifier is very high. In contrast, a comparator is designed to provide an output signal representing the voltage between the two input terminals of the comparator and thus, in normal use in a circuit generally some voltage exists between the input terminals of the comparator. However, due to the exterior circuits providing signals to the input terminals the input voltage can become too large and destroy the comparator circuit If the feedback for an operational amplifier is not functioning properly, the amplifier can also be destroyed if the input voltage becomes too large. The feedback may be non-operative be cause a large voltage is provided to the input at the same time as xe2x80x9cslew-ratexe2x80x9d-limiting is made on the output terminal of the operational amplifier or because the output terminal of the operational amplifier is connected to a too great load. Therefore, protection circuits are require both for operational amplifiers, and in particular, for comparators. Comparators do not, in conventional circuits, have a mechanism limiting the voltage between the input terminals.
It is an object of the present invention to provide a simple and efficient protection of the input terminals of an amplifier of differential type.
The two input terminals of a transistor-based amplifier of differential type are the base terminals of the input, active, amplifying transistors. The base-emitter junctions in the input transistors are protected by transistors connected as diodes in which the emitters and bases are connected to each other. The active pn-junctions in the protection transistors are the junctions between base and collector, which normally have a greater breakthrough voltage in the backward direction than the pn-junction between the base and the emitter. The protection transistors can advantageously be made in substantially the same way as the input transistors and have substantially the same electrical properties making it simple to introduce the protection in an electronic integrated circuit.
Thus an amplifier of differential type comprises two amplifier transistors. The input terminals of the amplifier are directly connected to the bases of each of the amplifier transistors for receiving the input voltage, the difference of which is to be produced and/or amplified and is obtained as an output voltage on an output terminal of the amplifier. The output voltage thus represents the voltage between the input terminals. Preferably, the output terminal is directly connected to the collector of one of the amplifier transistors.
For protection of the input terminals, and in particular, to prevent excessive currents from flowing through the amplifier transistors, protective diodes are connected directly in series with the emitter of each of the first transistors. Each of the protective diodes is connected to have the same direction or polarity as the diode formed by the pn-junction between the base and the emitter of the transistor to which it is connected. Furthermore, the protective diodes are formed by protective transistors, the emitter and the base of which are electrically connected to each other.
Generally, a protective diode is connected in series with that one of the pn-junctions in each of the amplifier transistors which has the lowest breakthrough voltage in the backward direction and has the same direction or polarity as the diode formed by the pn-junction of the first transistor having the lowest breakthrough voltage. The protective diodes each comprise a protective transistor of the same polarity type as the amplifier transistors, which in a differential amplifier generally are substantially identical or at least have substantially identical electrical characteristics or properties. That one of the first and second pn-junctions in each of the protective transistors which has the lowest breakthrough voltage in the backward direction is short-circuited by a electrical connection.
The amplifier transistors and the protective transistors can be npn-transistors of substantially the same kind or pnp-transistors of substantially the same kind.